Rhodium germanium film resistor



Jan. 2, 1962 w. J. Ma DONALD 3,015,587

RHODIUM GERMANIUM FILM RESISTOR Filed Sept. 5, 1958 IN V EN TOR.

WILLIAM J. MACDONALD p xc cmwa\ mud/Z6 ATTOR N EYS United 3,tll5,537RHQDTUM GERR EANIUM FllLli/i REfilSTtER William 3. Maclltonald,Littieton, Mass assignur to Technoiogy Instrument Corporation or Acton,a corporation of Massachusetts Filed Sept. 5, 1953, Ser. No. 75%,165 7Claims. (Cl. 117-213) This invention relates to electrical resistorelements and more particularly to vacuum deposited film resistors.

In the past various substances have been used in making precisionresistor elements the most common being metals or metal alloys, orvitreous carbon. However resistor elements of these materials becomeunstable when operated at high temperatures and are entirelyunsatisfactory in the range 250 to 300 C. Resistors capable of stableoperation in this temperature range have been made of stannous oxidecoated with silicon. However they are only of limited value as thesilicon coating prevents their use as variable resistors.

It is known that the noble metals are resistance stable at temperaturesas high as 300 C. Few, if any other metals are stable at suchtemperatures but often their oxides, nitrides and hydrides are. On theother hand, the oxides, nitrides and hydrides usually produceundesirable contact noise when used as variable resistors. Resistorelements made of noble metals also have disadvantages. Their resistivityis low and they have very high temperature coefiicients of resistance.These undesirable characteristics can be minimized by alloying noblemetals with less noble metals but in so doing high temperature stabilityis lost.

It is an object of my invention to overcome the above and analogousdisadvantages. Another object of my invention is to provide new andimproved resistor elements. It is a further object to provide a methodof producing resistor elements which are resistance stable attemperatures of 300 C., and have high resistivity and low temperaturecoetficients of resistance.

When resistor elements are used as variable resistors in potentiometersspecial problems arise. Each time the slider passes over the surface ofa variable resistor it causes a slight abrasion of the resistor element.In addition, contact between the slider and the resistor producesundesirable contact noise characteristics. Therefore, still anotherobject of my invention is to provide a resistor element which isabrasion resistant and which produces a minimal amount of noise whenused in a potentiometer.

Individually, rhodium has a low resistivity and a relatively highpositive temperature coefiicient of resistance whereas germanium hasalmost infinite resistivity and a relatively high negative temperaturecoefficient of resistance. A mixture of the two has surprising anddesirable characteristics. First of all, thin film may be obtainedhaving a range of between approximately 10 to 2090 ohms per square,depending upon the ratio of rhodium to germanium. Secondly, a filmcomprising a mixture of the two substances may be obtained having atemperature coefiicient of resistance of between approximately 235 and450 parts per million per C., as contrasted with pure rhodium which hasa value in excess of 1000 p.p.m./ C. and germanium which has a highnegative temperature coefiicient of resistance. Thirdly,rhodiumgermanium films are obtainable which are substantially resistancestable at temperatures as high as approximately 300 C.

These and other objects of my invention will best be understood andappreciated from the following description, when considered togetherwith the accompanying drawing which is a perspective view, partly brokenaway, of apparatus for working the invention.

In practicing my invention a film composed of a mixture of rhodium andgermanium is caused to deposit uniformly on the surface of a speciallyprepared substrate 13, after which the substrate with its film coatingis heat treated.

After cleaning and slightly abrading the substrate material 13, it ismounted in a vacuum chamber 12 on a holder 15, which holder is in turnconnected with a motor 16 in such a way as to permit the motor 16 torotate the substrate 13 in its holder 15. Directly in front andapproximately six inches away from the face of the substrate 13 ismounted a tungsten wire spiral 11 upon which rhodium has beenelectrolytically plated and to which germanium has been fused under ahydrogen atmosphere. This helix shaped wire ll with its rhodium andgermanium load is mounted so that an electric current can be passedthrough it. An electrically-energized tungsten heater 14 is mounted inclose relation to the substrate 13 and its holder 15.

Before beginning deposition the substrate 13 in its holder 15 is causedto rotate by means of the motor 16 and air is evacuated from the chamber12 to a pressure level of between approximately 5 10 and 1 l0- mm. Hg.Subsequently, by means of radiation from filament heaters 14 within thechamber 12, the substrate 13 is preheated to a temperature betweenapproximately 150 and 250 C. An electric current is then passed throughthe charged helix ll sufiicient to heat it above the melting points ofrhodium and germanium. As the two elements melt they immediatelyevaporate and the gaseous rhodium and germanium atoms impinge upon thesubstrate surface 113 depositing as a thin film. The electric current ispassed through the helix until all the rhodium and germanium have beenevaporated.

Upon completion of evaporation the substrate with its film coating iscooled and exposed to the air, following which it is subjected to heatat approximately 300 C. for a suitable period, generally between to 200hours to obtain electrical stability. The resulting heat treated rhodiumgermanium film is suitable for use as a precision electrical resistorelement at temperatures as high as approximately 300 C. When used in apotentiometer my rhodium-germanium resistor elements are capable of arotational life of approximately one million revolutions and generallyhave an equivalent noise resistance level, as measured by the apparatusdisclosed in US. Patent No. 2,778,994 substantially less than 100 ohmsENR.

In practicing my invention I prefer to use clean fused quartz as asubstrate material upon which the vaporized rhodium and germanium aredeposited. However, I may also use clean Pyrex glass or other materialssuch as soft glass or an inert ceramic as the substrate material.

Regardless of the substrate composition, it is important to cleanthoroughly the entire surface upon which the film is to be deposited.The same method is employed in cleaning quartz and Pyrex substrates. Thesurface is first polished and then abraded with an abrasive powder usingan air blast. Following an acid etch which removes surface stresses andsharp surface points: created by the blasting, the surface is Washedwith solvent solution. Mechanical removal of insoluble solids and aquick residue free dry thereby results. The same cleaning method is usedfor ceramic materials except that I omit the abrading step. Ceramicmaterials are too brittle and, therefore, will not abrade in the samemanner as quartz or Pyrex glass. The purpose of abrading is to increasethe surface area and thereby produce a greater resistance for a film ofgiven length and breads.

The substrate 13 shown in the accompanying drawing is circular in shapeso as to be suitable for use in a rotary potentiometer. However, it willbe understood that my invention is not necessarily restricted to suchuse, and

that the shape of the substrate may be varied as desired.

The amounts of the two elements carried by the helix are determined bythe film thickness desired to be produced and the ratio of the twoelements carried by the helix is determined by and is in the sameproportion as the ratio of the two elements in the film sought to bedeposited on the substrate. I have experimented with various ratiosincluding the following: (a) 88% rhodium, 12% germanium; (b) 70%rhodium, 30% germanium; and (c) 44% rhodium, 56% germanium. Theforegoing specific ratios all have performed satisfactorily, though the70% rhodium, 30% germanium ratio appears to be the most useful of thethree for commercial purposes.

The vaporized rhodium and germanium spreads out equally in alldirections from the helix. To achieve an evenly distributed film on thesubstrate it is of course best to position the helix directly in frontof the substrate. I have found 6 inches to be a satisfactory distancebetween the substrate and helix but it will be apparent to those skilledin the art that this distance may be varied as part of the means forcontrolling the thickness of the coating.

Proper heating of the substrate is of great importance to the success ofmy invention. The primary purpose of preheating the substrate in avacuum is to remove from it all moisture. It has been determined that amolecularly bonded water layer is normally present on the surface of thesubstrate and if it is not removed, the oxygen in the water will combinelater with the germanium and cause it to oxidize. It has been foundnecessary to heat the substrate above 100 C. in order to remove thiswater layer and to assure complete removal, the substrate must be heatedto a temperature of at least approximately 150. As heat in excess ofapproximately 250 C. tends to cause re-evaporation of the germaniumafter it has begun to deposit, care must be taken not to heat thesubstrate beyond that point. I have found 200 C. to be satisfactory inthe majority of cases. In addition, the effectiveness of the subsequentstabilizing heat treatment of the substrate and film is directlydependent upon proper heating during deposition as it has been foundthat the resistance of the rhodium-germanium film changes while it isheld at 300 C. and that the amount of this change is directlyproportional to the amount of heat applied during such deposition.

Here follow four examples of my invention, the first of which Idesignate as my preferred embodiment.

Example I In this preferred embodiment of my invention I first polisheda circular fused quartz disc to a degree comparable to a microscopeslide finish and then uniformly abraded it with a 1200 mesh aluminapowder using an air blast. A hydrofioric acid etch removed surfacestresses created by blasting and a trichloroethylene solution washingaccomplished removal of insoluble solids and produced a quick, residuefree dry. The substrate was then mounted on its holder in the vacuumchamber.

A tungsten helix upon which rhodium had been electrolytically plated andgermanium had been fused under a hydrogen atmosphere was prepared, thetwo elements being in a ratio of 70% rhodium and 30% germanium byweight. The prepared helix was mounted in the chamber six inches infront of the exposed substrate surface and the cover put over thechamber. Air was then withdrawn from the chamber to a vacuum of 1 10 mm.Hg after which rotation and heating of the substrate was begun. As thesubstrate in its holder revolved at 60 r.p.m., tungsten heaters adjacentto it raised its temperature to 200 C.

When the temperature of the substrate had reached 200 C. a current waspassed through the loaded tungsten helix sufiicient to heat its coatingabove the melting points of both rhodium and germanium. Current waspassed through the helix until its coating had completely evaporated; inthis preferred embodiment the total amount of the two elements on thehelix was sufficient to produce upon deposition, a film 200 angstromsthick. The substrate with its film coating was then cooled and exposedto the air.

At this point, after withdrawal from the vacuum chamher and before heattreatment the film had a resistance of approximately 625 ohms per squarevalue and a temperature coefficient of resistance of +150 parts permillion per degree centigrade. The film on its substrate was then heattreated. After exposure to heat of 300 C. for hours, the percentage ofresistance change was +100% and had a T.C. of +280 p.p.m./ C. After anadditional 600 hours of heat treating the additional percentage ofresistance change was +1% and the film had a T.C. of +282 p.p.m./ C.

Example II In this example a Pyrex substrate was prepared and cleaned inthe same manner as the quartz substrate of Example I. A tungsten helixwas prepared in the same manner as in Example I but with a coatingcomprising 88% rhodium and 12% germanium. The total quantity wassuflicient to produce a film 200 angstroms thick when subjected to thesame vacuum evaporation process as the film in Example I. The resistanceof this film upon withdrawal from the vacuum evaporation chamber andbefore heat treatment was approximately 490 ohms per square value and ithad a T.C. of +46 p.p.m./ C. After exposure to heat of 300 C. for 100hours, the percentage of resistance change was +.4% and T.C. was +450-p.p.m./ C. After an additional 400 hours of heat treatment thepercentage of additional resistance change was +.7% and T.C. wasstabilized at +450 p.p.m./ C.

Example 111 In this example a Pyrex substrate was prepared and cleanedin the same manner as the quartz substrate of Example I. A tungstenhelix was prepared in the same manner as in Example I but with a coatingcomprising 70% rhodium and 30% germanium. The total quantity wassufircient to produce a film 200 angstroms thick when subjected to thesame vacuum evaporation process as the film in Example I. The resistanceof this film upon withdrawal from the vacuum evaporation chamber andbefore heat treatment was approximately 745 ohms per square value and ithad a T.C. of 0 p.p.m./ C. After exposure to heat of 300 C. for 100hours the percentage of resistance change was 19% and the T.C. was +270p.p.m./ C. After an additional 400 hours of heat treatment thepercentage of additional resistance change was -2% and the T.C. wasstabilized at +270 p.p.m./ C.

Example IV In this example a Pyrex substrate was prepared and cleaned inthe same manner as the quartz substrate of Example I. A tungsten helixwas prepared in the same manner as in Example I but with a coatingcomprising 44% rhodium and 56% germanium. The total quantity wassufficient to produce a film 200 angstroms thick when subjected to thesame vacuum evaporation process as the film in Example I. The resistanceof this film upon withdrawal from the vacuum evaporation chamber andbefore heat treatment was approximately 10,000 ohms per square value andit had a T.C. of 100 p.p.m./ C. After exposure to heat of 300 C. for 100hours the percentage of resistance change was 10% and the T.C. was 260p.p.m./ C. After "an additional 400 hours of heat treatment thepercentage of additional resistance change was 27% and the T.C. wasstabilized at 260 p.p.m./ C.

In producing my invention the proportions of the two elements can bevaried greatly by controlling the amounts of the two elements which areplaced upon the tungsten helix, practical limits being set by therequirements for which the resistor element is to be used. Heat treatment is essential in order to stabilize the film resistors and therebymake them useful for commercial and experimental purposes.

As might be expected, variations in the thickness of the film result indifferent resistance values for a given width and length of film. Thickcoatings yield low resistance films, while thin coatings yield films ofhigh resistance. Control of film thickness is achieved through controlof the amount of the two elements loaded on the helix and to a lesserextent by the distance in the vacuum chamber between the helix and thesubstrate.

Rotation of the holder during evaporation is not "absolutely necessary.It is a preferred step which contributes to greater quality control, butcan be omitted without harm.

Furthermore, as seen from the previous examples, different substratematerials will produce variations in both the resistance of the filmupon withdrawal from vacuum and before heating and the amount ofresistance change as a result of heating. I have found a quartzsubstrate to be most satisfactory for my own purposes.

The method described hereinabove may be employed to deposit and formfilms comprising molecular mixtures of substances other than rhodium andgermanium. Thus, for example, I have deposited a film comprising amixture of nickel, chromium and germanium by the same method and usingthe same apparatus. All three elements were fused onto the helix 11,with the nickel and chromium prior to application to the helix takingthe form of Nichrome alloy.

In addition to those enumerated above certain minor variations of myinvention will be apparent to those skilled in the art. Hence it is notmy intention to confine the invention to the precise forms hereindescribed but rather to limit it in terms of the appended claims.

Having thus described and disclosed my invention and the method by whichit can be produced, what I claim as new and desire to secure by LettersPatent of the United States is:

1. A process for producing an electrical resistor element comprising thesteps of providing as a substrate a piece of rigid electricallyinsulating material having a planar surface, cleaning and slightlyabrading said planar surface, depositing on said planar surface a thinfilm comprising rhodium and germanium in predetermined relative amounts,and thereafter subjecting said substrate and film to heat treatment atan elevated temperature for an extended period of time.

2. A process as defined by claim 1 further including the step of heatingsaid substrate to a predetermined temperature in excess of roomtemperature preliminary to depositing such film, said predeterminedtemperature being less than approximately 250 C. and more thanapproximately 150 C.

3. A process for producing an electrical resistor element comprising thesteps of providing as a substrate a piece of rigid electricallyinsulating material having a planar surface, cleaning and slightlyabrading said planar surface, heating said substrate to a predeterminedtemperature in excess of room temperature preliminary to depositing saidfilm, said predetermined temperature being less than approximately 250C. and more than approximately 150 C., and depositing on said planarsurface a thin film comprising rhodium and germanium in predeterminedrelative amounts, said deposition being accomplished by means ofevaporation under a vacuum of between approximately 5 10- and 1 10 saidevaporation of rhodium and germanium being accomplished by passing anelectric current through a spiral tungsten filament coated with rhodiumand germanium, said heating and said passing of an electric currentthrough said tungsten filament being continued until approximately allthe rhodium and germanium have been evaporated from said tungstenspiral, said evaporation being followed by cooling of the substrate withits coating of rhodiumgerm-anium film and removal from the vacuumchamber.

4. A process as defined by claim 3 followed by the step of subjectingsaid substrate with its coating of rhodium and germanium to heat ofapproximately 300 C. for between approximately and 200 hours in air.

5. A process as defined by claim 3 further including the step of slowlyrevolving the said substrate during deposition.

6. A process as defined by claim 4 further including the step of slowlyrevolving said substrate during deposition.

7. A process for producing a film comprising a molecular mixture ofrhodium and germanium, comprising the steps of providing a substratehaving a clean planar surface, providing an electrically conductivefilament, coating said filament with predetermined quantities of rhodiumand germanium, placing said substrate in a vacuum chamber, preheatingsaid substrate under vacuum to a temperature of at least approximatelyC. to remove all surface moisture therefrom, thereafter while saidsubstrate is still at a temperature of 150 C. passing an electriccurrent through said filament to heat said filament to a temperaturesufficient to cause evaporation of said rhodium and germanium,discontinuing said electric current after approximately all of saidrhodium and germanium have been evaporated off of said filament, andcooling said substrate prior to removal from said chamber.

References Cited in the file of this patent UNITED STATES PATENTS2,097,140 Wohrman et al. Oct. 26, 1937 2,341,219 Jones Feb. 8, 19442,423,476 Billings July 8, 1947 2,552,626 Fisher et a1 May 15, 19512,629,800 Pearsons Feb. 24,, 1953 2,778,743 Bowman Jan. 22, 19572,789,187 Romer Apr. 16, 1957 2,847,325 Riseman et a1 Aug. 12, 19582,855,493 Tierman Oct. 7, 1958

1. A PROCESS FOR REDUCING AN ELECTRICAL RESISTOR ELEMENT COMPRISING THESTEPS OF PROVIDING AS A SUBSTRATE A PIECE OF RIGID ELECTRICALLYINSULATING MATERIAL HAVING A PLANAR SURFACE, CLEANING AND SLIGHTLYABRADING SAID PLANAR SURFACE, DEPOSITING ON SAID PLANAR SURFACE A THINFILM COMPRISING RHODIUM AND GERMANIUM IN PREDETERMINED RELATIVE AMOUNTS,AND THEREAFTER SUBJECTING SAID SUBSTRATE AND FILM TO HEAT TREATMENT ATAN ELEVATED TEMPERATURE FOR AN EXTENDED PERIOD OF TIME.